Lecture 10:Static & Final
Kenya 2005
MyMath Example
public class MyMath {
public double PI = 3.14159;
public double square (double x) {
return x * x;
}
public static void main(String[ ] args) {
MyMath m = new MyMath();
System.out.println("m: value of PI is " +
m.PI);
System.out.println("m: square of 5 is " +
m.square(5));
}
MyMath n = new MyMath();
System.out.println("n: value of PI is " +
n.PI);
System.out.println("n: square of 5 is " +
n.square(5));
}
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Objects Review
• In Example 1, to calculate the square of
5 we need to create an instance of
MyMath class:
MyMath m = new MyMath();
• Then we invoke it’s square() method
with the argument 5:
m.square(5);
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MyMath Output
•
The results of invoking square() method on
instances m and n are the same:
m:
m:
n:
n:
value of PI
square of 5
value of PI
square of 5
is
is
is
is
3.14159
25
3.14159
25
•
square() behaves the same no matter
which instance it is called on.
•
So . . . why not have one square() method
for the entire class?
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Also . . .
• The value of
PI = 3.14159 is the
same for all instances of MyMath class.
• Why do we need to store a value of PI
separately for each instance of
MyMath?
• Instead, can we have only one common
value of PI for the whole MyMath
class?
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MyMath with static
public class MyMath {
// add keyword "static" to field declaration
public static double PI = 3.14159;
// add keyword "static" to method declaration
public static double square (double x) {
return x * x;
}
// main method is always declared "static"
public static void main( String[ ] args) {
// MyMath m = new MyMath(); - No longer need this
line!
// MyMath n = new MyMath(); - No longer need this
line!
}
// Now invoke square() method on the MyMath class
System.out.println("Value of PI is " + MyMath.PI);
System.out.println("Square of 5 is" +
MyMath.square(5));
}
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Static Pi Field
• We added word static to the
declaration of the final variable PI:
public static double PI = 3.14159;
• It means that now we have only one
value of variable PI for all instances of
MyMath class; PI is now a class data
field
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The final keyword
•
We declared PI as
public static double PI = 3.14159;
but this does not prevent changing its value:
MyMath.PI = 999999999;
•
We use keyword final to denote a constant:
public static final double PI = 3.14159;
•
Once we declare a variable to be final, it's
value can no longer be changed!
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Final References
•
Consider this final reference to a Point:
public static final Point ORIGIN =
new Point(0,0);
•
This prevents changing the reference ORIGIN:
MyMath.ORIGIN = new Point(3, 4);
•
BUT! You can still call methods on ORIGIN that
change the state of ORIGIN.
MyMath.ORIGIN.setX(4);
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MyMath with static & final
public class MyMath {
// add keyword final to field declaration
public static final double PI = 3.14159;
public static double square (double x) {
return x * x;
}
public static void main( String[ ] args) {
System.out.println("Value of PI is " +
MyMath.PI);
System.out.println("Square of 5: " +
MyMath.square(5));
}
}
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Static Fields
• Only one instance of a static field data
for the entire class, not one per object.
• "static" is a historic keyword from C/C++
• "Class fields" is a better term
–
As opposed to "instance fields"
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Static Square Method
•
We also added the word "static" to the
declaration of the method square():
public static double square(double x) {
return x * x;
}
•
Now the method square() is shared by all
instances of the class—only one square
method for the class, not one for each instance.
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Static Methods
• Static methods do not operate on a
specific instance of their class
• Have access only to static fields and
methods of the class
–
Cannot access non-static ones
• "Class methods" is a better term
–
As opposed to "instance methods"
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Java's Math Class
•
•
•
Let's take a look at Java's Math class in the API
You cannot create an instance of the Math Class,
it's just a place to store useful static methods
All the methods and fields are static:
Math.sqrt(16)
Math.PI
Math.abs(-3)
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Static Field Examples
•
Constants used by a class
(usually used with final keyword
- We only need to have one per class; don’t need one in
each object- Look at the following declaration:
public static final double TEMP_CONVERT= 1.8;
–
If the variable TEMP_CONVERT is in class
Temperature, it is invoked by:
double t = Temperature.TEMP_CONVERT * temp;
–
Constants are all capital letters by tradition (C, C++) for example:
PI , TEMP_CONVERT etc.
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Static Method Examples
• For methods that use only the arguments and
therefore do not operate on an instance
public static double pow(double b, double p)
// Math class, takes b to the p power
• For methods that only need static data fields
• We use the static key word on the main
method in the class that starts the program
–
No objects exist yet for the main method to
operate on!
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POP QUIZ
Should it be static or non-static?
• Speed of light field
• getName() method in a Person class
• A sum method that returns the
resulting of adding both its arguments
• Width data field in a Rectangle class
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POP QUIZ
Should it be static or non-static?
• A field that stores the speed of lightstatic
• getName() method in a Person class non
• A sum method that returns the
•
resulting of adding both its arguments
static
Width data field in a Rectangle class non
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